1. Field of the Invention
The present invention relates to electronic communication technology and more particularly to fiber optic systems with modulation of the light carriers with microwave frequencies for single-mode fibers.
2. Description of Related Art
Fiber-optic networks demand transmission methods that offer flexibility and the efficient exploitation of bandwidth of existing network assets, such as existing conventional single-mode fiber. Wavelength-division multiplexing (WDM) has been successfully used to transmit multiple optical carriers on a single fiber. Subcarrier multiplexing (SCM) is one of the few techniques that can accommodate the multi-format array of transmission protocols and modulation formats expected to be carried on networks. SCM can be combined with WDM to greatly increase the transmission capacity of a single fiber.
One challenge to the implementation of SCM has been the limitation on transmission distance. Normally, operation with laser light carrier wavelengths of 1550 nm permits the use of erbium-doped fiber amplifiers (EDFAs) to overcome transmission loss, but the dispersion-limited maximum transmission distance of an SCM system is dependent on its total modulation bandwidth.
Another traditional drawback to SCM has been the complexity of demodulation schemes. Hill and Olshansky demonstrated SCM using coherent detection, but this technique is too impractical to use in a telecommunications environment, and downconversion of microwave subcarriers requires phase matching to the transmitter. (P. Hill, et al., "Bandwidth Efficient Transmission of 4 Gb/s on Two Microwave QPSK Subcarriers Over a 48 km Optical Link", IEEE Photonics Technology Letters, vol. 2, no. 7, July 1990, pp. 510-512; and P. Hill, et al., "8 Gb/s Subcarrier Multiplexed Coherent Lightwave System", IEEE Photonics Technology Letters, vol. 3, no. 8, August 1991, pp. 764-766.)
Although numerous experimental high-speed SCM systems have been demonstrated, the technology has not been embraced by the telecommunications industry. Ordinary single-mode fibers have been installed all over the world, and conventional long-haul fiber systems have thus far relied on dispersion-shifted fiber in answer to the dispersion problem that accompanies carrier wavelengths of 1550 nm. Typical SCM systems have receivers that use coherent detection. Greenhalgh, et al., demonstrated an optical pre-filtering technique for subcarrier demultiplexing in a low bandwidth SCM link but did not explain how that could be used to reduce the effects of dispersion. (P. A. Greenhalgh, et al., "Optical prefiltering in subcarrier systems", SPIE, vol. 1790, Analog Photonics (1992), pp. 76-84.)